Currently an open standard for a service called Push-to-talk over Cellular (PoC) or Instant-Talk-over-Cellular (IToC) is developed, which service will be applied in terminals in cellular telecommunications systems such as GSM, EDGE, UMTS and CDMA systems. A list of abbreviations is provided at the end of the specification.
Push-to-talk over Cellular (PoC) is basically a “walkie-talkie” service in a cellular telecommunications system. PoC enabled terminals will most likely be equipped with a PoC-button. This PoC button may either be a designated hardware button, one of the existing buttons in the standard keypad or a software controlled button, eg. a button defined on a pressure sensitive display or the like. When this button is pressed the terminal instantly connects you to a friend, a family member or a group of people of your choice, that is no number taking is required. Like a “walkie-talkie” the PoC service is half-duplex, although full duplex may be available at a later stage of development. It is important to have low setup delay in order to allow for the user to start speaking immediately after pressing the button.
FIG. 1 shows an example of the setup of a PoC service architecture in a general telecommunications network 1 comprising a PoC server 2 that administers the PoC service. The PoC server 2 is located in a service network 3, which is associated with a number of Core Networks (CN) 4 each comprising a Service GPRS Support Node 5 (SGSN) and a Gateway GPRS Support Node 6 (GGSN). At least one Radio Access Network 7 (RAN) in turn comprising at least one Base Station Subsystem 8 (BSS), is associated with each core network, which RAN communicates with a number of User Terminals (UT1-UT5) via suitable radio telecommunications protocols. As described above, the PoC service allows half-duplex messages from one user terminal to one or more other terminals in a simple manner.
FIG. 2 shows a more detailed illustration of the setup of a PoC service architecture. The different arrows indicate different types of messages that are sent in the service. The PoC service comprises a PoC server 2 that administers media transfer, a group/list manager server 9 that administers group/contacts handling, a presence server 10, and an IP Multimedia Subsystem core 11 (IMS) that administers session control signalling with the user terminal UT.
The IMS is a system for supporting IP based multimedia services, comprising a Home Subscriber Server (HSS), and at least one Call Session Control Function (CSCF). The HSS maintains the subscriber profile for the CS (Circuit Switched) domain, PS (Packet Switched) domain and IP Multimedia Subsystem. The CSCF in turn provides the IP multimedia basic SIP session handling. It has three functional roles—a “service CSCF” (S-CSCF), an “interrogating CSCF” (I-CSCF) and a “proxy CSCF” (P-CSCF). The P-CSCF is the first point of contact for the mobile station. It proxies the SIP messages towards the home network for the subscriber. The P-CSCF may perform number internationalisation, and it may enforce policy in the GGSN relating the handling of the bearer traffic for IP multimedia. The I-CSCF is the entry point to the home network. The other networks will use a Domain Name Server (DNS) to route the messages to the home network, which will lead the SIP signalling to the I-CSCF. The I-CSCF contacts the HSS to gain the address of the S-CSCF and forwards the SIP messages to the S-CSCF. The S-CSCF is the SIP proxy which provides the access to the operator provided services to the end-user.
FIG. 3 shows an example of a PoC communication (early media setup) between a user 1 and a user 2 where user 1 is the originating part. In the figure each arrow indicates a message and its direction. As is indicated above BSS 8 is a base station subsystem and CN 4 is a core network. For clarity reasons the telecommunications network in FIG. 3 has been divided into two parts, an originating side and a terminating side, respectively; however the BSS 8 and CN 4 on the terminating side may be the same as the BSS 8 and CN 4 on the originating side. UT1 is the terminal of user 1 and UT2 the terminal of user 2. Throughout the application, actions relating to the PoC service are marked with bold reference numbers. The PoC service is initiated in that user 1 selects one or more communication partners or receivers in his/her PoC contacts list, action step 20. Thereafter user 1 pushes the PoC button, process step 21, and the terminal UT1 sends a PoC (floor) request signal 22 to the PoC server 2 via the BSS and the CN. If the PoC service is available (the floor is free), i.e. no one of the receivers already has initiated a PoC message, then the PoC server sends a PoC response signal 23 to UT1 which indicates that UT1 may send a PoC message to selected receivers (in this case user 2). UT1 alerts user 1 that the transmission can be affected and that he/she can begin to talk, action step 24. The message 25 is speech coded and packeted into user data packets (Ud packets) 26 that are sent to the PoC server for distribution to the selected receivers.
On the terminating side, UT2 first receives a PoC request 27 indicating a PoC message 28 from user 1. User 2 accepts the PoC message by pressing an accept button 29 or the like which triggers a PoC accept message 30 from U12. As a response to the PoC accept message 30 the PoC server forwards the PoC message 26 from user 1. When user 2 has received the message 31, 32 he/she may choose to make a response 33 to the message. A response from user 2 is initiated by pushing the PoC button 33 on UT2 whereby a PoC request 34 is sent to the PoC server, just like when the communication first was initiated by user 1. When the PoC server receives the PoC request, it checks if the session (floor) is free, i.e. no one else in the same PoC group already has submitted a PoC request. Obviously, there will be no problem to receive a positive PoC response 35 in the present situation with only two users involved, but in PoC groups with a plurality of users, a user might have to send several PoC requests 34 before he/she will receive a positive PoC response 35 and the possibility to send a reply message 37 to the other users in the group. The remaining steps 38 to 44 in FIG. 3 are performed in the same manner as above.
In an alternative PoC communication setup called “late media” the steps 27 to 30 are performed between the steps 22 and 23. In this setup, user 2 is located and alerted before user 1 is allowed to submit his/her message.
Due to the facts that: the service is delay sensitive, the service is run over a non real time system, and that the amount of signalling is significant, system delay becomes a critical issue. As different types of delays in this type of service add up, it is of great importance to minimize all sources of delay so that the total perceived delay is as low as possible. Even delays in the order of 100 ms become important to reduce.
One contribution to these delay times originates from setting up radio connections between the user terminals and the Base Station sub-System (BSS) and vice versa. Before any data can be transmitted over the air interface in the mobile communication systems, states must be established in the user terminals and in the base stations, and so called radio bearers need to be set up. For example, in the GSM/GPRS system a radio connection referred to as Temporary Block Flow (TBF) must be established between the user terminal and the BSS in order for data to be exchanged. The TBF includes e.g. allocation of time slots. The TBF in the direction user terminal to BSS is referred to as uplink TBF (UL TBF) and in the reverse direction, BSS to user terminal, the TBF is referred to as downlink TBF (DL TBF).
Today there are means to delay the tear down of the TBFs, which are usually tore down when there is no data in the sending buffers located in the terminal or in the BSS, so called TBF prolonging. The prolonging of the DL TBF is referred to as Delayed Release of DL TBF, and the prolonging of the UL TBF is referred to as Extended UL TBF Mode. This prolonging reduces delay times for setting up a new radio connection when new data is placed in the sending buffer shortly after it was emptied.
Further, there exists a feature called Early Setup of a DL TBF. When approaching the end of an UL TBF the BSS will setup a DL TBF even if there is no data in the sending buffer of the BSS and will prolong this TBF for a while. So when new data arrives from a Core Network, the TBF is in place and a delay due to setting up of a radio connection is avoided.